When you draw a circle with a pair of compasses, you use the point on one of the arms to prick a dot in the centre of the circle. The Greek word kentron meant ‘sharp point’, specifically the one on a pair of compasses, and for this reason the words descended from it, including the English centre, came to refer to the centre of a circle. What is now the American form center is in fact the older spelling, found in the works of Shakespeare. It was Dr Johnson's dictionary in 1755 that established centre as the preferred British spelling. Concentrate (mid 17th century) literally ‘centre together’ reached English via Latin.